Low-Cost Halide Electrolytes Li2+xHf1–xFexCl6 with Superior Ionic Conductivities for All-Solid-State Lithium–Metal Based Batteries

All-solid-state batteries (ASSBs) employing inorganic solid electrolytes have been considered as promising candidates for next generation energy storage owing to their intrinsic safety performance and high energy density. Designing highly ionically conductive and (electro)chemically stable inorganic solid electrolytes utilizing cost-effective materials is of vital importance for the development of practical ASSBs. Herein, we report a series of new lithium-conducting superionic halides Li2+xHf1–xFexCl6 that are free of rare-earth elements with high ionic conductivities up to 0.91 mS cm–1 at 30 °C by aliovalent substitution with low-cost and earth-abundant Fe elements. By means of complementary characterization techniques and bond-valence site energy (BVSE) calculations, we gain insights into the influence of aliovalent doping engineering on the local structural environment and the underlying lithium-ion transport properties of Fe3+-substituted Li2HfCl6. Importantly, it is demonstrated that the prevalently existent distortion of octahedral structure and redistribution of the lithium ion induced by the aliovalent substitution strongly benefits the transport properties. Notably, the formation of infinitely 3D connected lithium-ion migration pathways comprised of a directly connected face-sharing octahedron along the c direction is revealed by structural analysis and theoretical calculations. Additionally, owing to the intrinsic oxidation tolerance of Fe3+-substituted Li2HfCl6, the fabricated bulk-type ASSBs with uncoated LiCoO2 deliver an outstanding electrochemical performance.